Known in the art is an acoustic surface wave filter having a piezoelectric substrate, an input and output acoustic surface wave interdigital transducers arranged in one acoustic channel (cf. R. H. Tancrell and M. G. Holland "Acoustic Surface Wave Filter Proc. IEEE," vol.59, No.3, p.p.393-409. Mar.1971).
In this filter, one of the transducers, e.g. the input transducer is apodized, i.e. the length of the overlapping portions of the adjacent electrodes of this filter is changed in accordance with a specified law of amplitude modulation in the impolse response of the filter. The other (output) transducer is a wide-band and non-apodized device, i.e. having the same length of the overlapping portions of all electrodes. In such a filter, the amplitude-frequency response is provided only by one transducer, namely, by the apodized transducer, namely, by the apodized transducer so that it is impossible to achieve a high level of suppression of the signal beyond the filter pass band. Therefore, the shape of its amplitude-frequency response is unsatisfactory.
Also known in the art is an acoustic surface wave filter comprising an input and output acoustic surface wave transducers arranged on a piezoelectric substrate in spaced acoustic channels and a coupling element connecting the acoustic channels of the input and output transducers, said element being arranged on the same piezoelectric substrate (cf. J. M. Deacop, J. Heighway, I. A. Jenkins "Multistrip Coupler in Acoustic Surface Wave Filters Electr. Let. vol.9, No.10, p.235, 1973)." In this filter, both transducers are apodized while the coupling element is made in the form of a multistrip system of electrodes.
In the above described filter the high-frequency signal fed to the input transducer is converted into acoustic surface waves, which run to the multistrip system of electrodes and are transformed there to an acoustic signal of the output transducer where they are again transformed to a high-frequency signal. During the conversion of the acoustic surface waves the acoustic surface wave front is levelled on the electrode system along the beam aperture. This makes it possible to provide the amplitude-frequency response of the filter by apodizing both input and output transducers. The resultant amplitude-frequency response of such a filter is the product of the amplitude-frequency responses in the input and output transducers. This makes it possible to improve the shape of the resultant amplitude-frequency response of the filter. However, the level of suppression of the signal outside of the pass band of this filter is inadequate. Therefore, the shape of the amplitude-frequency response of this filter, like that of the filter described above, is unsatisfactory. In addition, a very limited amount of materials can be used for making such a filter, since the multistrip system of electrodes coupling the acoustic channels of the transducers can be realized only in materials having a high electromechanical coupling constant (e.g. lithium niobate). In materials having a relatively low electromechanical coupling constant (e.g. in quartz), the number of electrodes in the multistrip system is unacceptably large.